In a conventional internal combustion engine of the spark ignition type, for obtaining a stable idle operation and for preventing "knocking", the degree of advance of an ignition timing during the idle condition is usually maintained to be small. Thus, the combustion efficiency during the idle condition is low. In this case, the rotational speed of the engine in the idle condition is apt to decrease due to the load of the engine which is generated when auxiliary units of the engine, for example, an alternator, a power steering mechanism or an air conditioning apparatus are operating. Due to such low rotational speed occurring during the idle condition, an "engine stall" and/or an "over-heating" condition will easily take place.